1976 El Camino lose of power

My son tells me: he was driving down a dirt road around 60mph and everthing but the stereo shut down. The headlights were bright, running fine and then just nothing. He has the stereo wired seperatly to the battery. There is no juice to any lights, starter, etc. Just the stereo. When jumping, it remains dead. Battery connections are clean and tight. Battery, tuneup, wires, plugs, cap, roter, all less than 10,000 and 7 months.